Once the process is complete, we now have to repartition the drive in your Mac to allow for space for the macOS installation. Open up Disk Utility and select your drive and click Partition button in the toolbar. You should get a menu as shown below. Resize the new partition with the format OS X Extended (Journaled) and select the size. Note that the size that you enter here is the size of the current partition. So, putting in 70GB would mean that your OS X partition would be resized to 70GB and the rest will be for your new macOS installation.

Now that the partition process is complete and the bootable USB drive is ready too, reboot your Mac. Press and hold the Option key while the Mac boots and you will be presented with the startup manager. Select your USB drive here as a boot device and it should load up the macOS installer. During the installation process, make sure that you select the new partition you created so that you can dual boot both operating systems.

The benefit of dual booting is that you get to keep your stable copy of OS X running while you can test a beta version of macOS. It can also help in making sure that all your apps work fine with the new macOS before taking the plunge and removing OS X for good from your Mac.